This invention relates generally to semiconductor devices and more particularly to Gallium Arsenide (GaAs) field-effect transistors (FETs).
GaAs Metal-Semiconductor Field-Effect Transistors (MESFETS) are well known devices for providing amplification at microwave frequencies, high-speed digital switching, and various other functions. The use of microwave devices in satellite-based and wireless communications has grown in recent years. There is large market in such applications for MESFETs having very high power capability per unit transistor surface area, expressed in watts/mm2. As the power capability or output of transistors improves, a single transistor can provide the power which in previous generations was provided by multiple transistors, which provides a cost, volume, and weight savings. The greater the power capability of a transistor, the broader its potential applications, and the larger the potential market. Thus, there has been a great deal of activity directed toward improving the performance of transistors for military, industrial and commercial applications.
A conventional GaAs MESFET uses a metal electrode in direct contact with a doped GaAs channel region to form a Schottky gate junction. A voltage applied to the gate electrode or junction influences the channel carrier density in the vicinity of the gate electrode, so that current flow from drain to source through the channel region, under the impetus of a drain-to-source voltage, can be modulated by variation of the voltage applied between the gate electrode and one of the other electrodes of the FET. This modulation or control is what allows the MESFET to provide its amplification and/or switching functions.
It has long been known that during the dynamic performance of an FET used in large-signal applications, the instantaneous source-to-drain voltage makes an excursion from a relatively high value to a small value, and that the associated instantaneous drain-to-source current makes an excursion in the opposite direction. In general, the maximum allowable drain-to source voltage must be limited so as not to exceed the breakdown voltage of the transistor. The breakdown voltage is determined by the structural parameters of the transistor, including such factors as the spacing between the gate electrode and the drain electrode. Other such parameters include the breakdown field of the substrate material itself, as for example germanium, silicon, gallium arsenide, diamond, and the like, the doping levels of the various portions of the structure, the dimensions of the doped portions, and the detailed shape of the gate electrode and adjoining material. The breakdown field of a material is related to its bandgapxe2x80x94this means that materials with larger bandgaps than GaAs would be helpful in realizing transistors with higher breakdown voltages. However, breakdown voltage is only one of the parameters which is important in producing a transistor for microwave and switching applications, since the operating speed of the device is equally, or possibly more important, as well as the amount of current which can be switched by a given size device.
A known approach to the fabrication of GaAs MESFETs is described in A New Refractory Self-Aligned Gate Technology for GaAs Microwave Power FET""s and MMIC""s, by Geissberger et al., published in IEEE Transactions on Electron Devices, Vol 35, No. 5, May 1988. As described therein, a gate electrode overlies the channel and forms a Schottky junction, asymmetrically located relative to the source and drain. In order to reduce resistance of the gate in a direction transverse to the direction of source-to-drain current, a titanium-gold (Ti/Au) overlay is placed over the gate electrode. The Ti/Au material is more conductive than the material of the gate electrode, and its transverse dimensions are also greater, so its resistance is much less than that of the gate electrode.
One approach to improving power capability of a high-power gallium-arsenide MESFET, attributable to burnout near pinchoff, involves the recessing of the gate electrode within a trough or depression in the GaAs channel region, but this method has little effect on breakdown during instantaneous open-channel conditions. The open-channel burnout was then found to be improved by extending the extent of the trough or depression on either side of the gate electrode. [dcm1] The recessed gate is effective in increasing the power-handling capability of a GaAs MESFET, but requires more complex procedures during manufacture than might be desired for lowest cost.
Another approach to improving the power capability of a planar GaAs MESFET, without recourse to a recessed-gate structure, is described in U.S. Pat. No. 5,565,696, issued Oct. 15, 1996 in the, name of Miller et al. As described by Miller et al., the transistor includes an ion-implanted nxe2x88x92 guard region surrounding the n+ drain electrode, to thereby decrease the likelihood of breakdown of the drain-to-substrate or drain-to-subchannel junction. This transistor also includes an oversize conductive plate overlying that portion of the gate electrode forming the Shottky junction, for reducing the resistance of the gate to drive signals.
Another approach is described in U.S. Pat. No. 6,005,267, issued Dec. 21, 1999 in the name of Griffin et al. As described by Griffin et al., it was known to increase the gate-to-drain breakdown voltage of a planar GaAs MESFET by increasing the spacing between the gate electrode and the drain. While effective at increasing the breakdown voltage, the additional length (in the source-to-drain conduction direction) of channel introduced additional resistance into the source-to-drain path, and this additional resistance, in turn, tended to limit the ON-state or maximum current. Thus, the increase in power capability which might have been allowed by the increased breakdown voltage was mitigated by the decrease in current capacity. The Griffin et al. solution to this problem was to add an additional insulated electrode on the channel at a location lying between the gate and the drain, and to apply a sample of the signal to be amplified or switched to this additional electrode to modulate the ON-state resistance, and thereby at least partially overcome the effects of the resistance of the channel. The sample of the signal which is applied to the additional electrode is produced by means of a power divider and impedance transformers.
An article entitled Novel High Power AlGaAs/GaAs HFET with a Field-Modulating Plate Operated at 35 V Drain Voltage, by Asano et al, and published by the IEEE in 1998, describes a recessed-gate MESFET with a field plate similar to the one described by Griffin et al. located in the recess.
Improved planar MESFET power handling capability is desired.
A gallium-arsenide MESFET (MEtal-Semiconductor Field-Effect Transistor)includes a source, a gate, and a drain. The transistor comprises[dcm2] a substrate defining a planar surface, and a doped channel region in the planar surface of a given thickness. The channel region is elongated in the drain-to-source conduction direction and defines source and drain ends. An electrically conductive gate conductor defines upper and lower surfaces. The gate conductor overlies a portion of the channel region, and the gate conductor has its lower surface in contact with the channel region, to thereby form a Schottky junction. The gate conductor has first and second edges spaced apart in the source-drain conduction direction. According to an aspect of the invention, the second edge of the gate conductor is spaced about 1.8 microns from the drain end of the channel region. A source is electrically coupled to the source end of the channel region, and a drain is electrically coupled to the drain end of the channel region. The conductive electrodes of the source and/or drain may be directly connected to the channel, or they may be connected by means of intermediary semiconductor regions. Pursuant to a further aspect of the invention, an electrically conductive field plate is mechanically and electrically connected to the upper surface of the gate conductor. The field plate extends about 1.3 microns from the second edge of the gate conductor toward the second end of the channel region, and is electrically isolated from the channel region in regions other than that of the gate conductor. The electrical isolation is provided by an insulating substance having a thickness of about 100 nm and a dielectric constant (xcex5R) of about 5. In a particular embodiment, the insulating substance comprises silicon oxynitride (SiON). In another avatar, at least one of the source and drain comprises an n-doped semiconductor region. In a more particular version of this avatar, the n-doped semiconductor region is doped to about 1018 donors/cc. In yet another hypostasis of the invention, at least one of the source and drain further comprises an intrinsic semiconductor region.
A particularly advantageous objectification of the invention parallels first, second and third FET elements, with common drain structure for the first and second FET elements, and common source structure for the second and third FET elements.